Grid to compensate for astigmatic quadrupolar lens



v Feb. 24, 1970 J. HAskER' 3,

GRID TO COMPENSATE FOR ASTIGMATIC QUADRUPOLAR LENS Filed Dec. 6, 1968 3 Sheets-Sheet 1 INVENTOR. JAN HASKER Feb. 24, 1970 J. HASKER 3,497,763

7 GRID T0 COMPENSATE FOR ASTIGMATIC QUADRUPOLAR LENS Filed Dec. 6, 1968 s Sheets-Sheet 2 Feb. 24, 1970 J. HASKER 3,

. GRID '10 COMPENSATE FOR ASTIGMATIC QUADRUPOLAR LENS Filed D90. 6, 1968 3 Sheets-Sheet 5 INVENTOR.

JAN HASKER Ass -r United States Patent Ot'fice 3,497,763 COMPENSATE FOR ASTIGMATIC QUADRUPOLAR LENS Jan Hasker, Emmasingel, Eindhoven, Netherlands, assignor, by mesne assignments, to U.S. Philips Corporation, New York, N.Y., a corporation of Delaware Filed Dec. 6, 1968, Ser. No. 781,746 Claims priority, application Netherlands, Dec. 22, 1967, 6717636 Int. Cl. H01j 29/56 US. Cl. 31531 GRID TO Claims ABSTRACT OF THE DISCLOSURE The invention relates to an arrangement having a cathode-ray tube in which an electron beam produced by an electron gun comprising at least a cathode, a first grid and a last grid acting as an acceleration electrode is substantially focussed on a target with the aid of a focussing lens, which arrangement includes means for deflecting the electron beam in a first direction, means for deflecting the electron beam in a second direction substantially at right angles to the said first direction and a quadrupolar lens for deflecting amplification.

Such an arrangement may be used, for example, for reproducing television signals or for reproducing signals in an oscillograph. Due to the presence of the quadrupolar lens for deflection amplification, such an arrange ment requires only a comparatively small amount of deflection energy to obtain an adequate deflection of the electron beam. Due to the presence of such a quadrupolar lens, very small signals can be satisfactorily reproduced in an oscillograph. The quadrupolar lens acts as an astigmatic lens. In one direction of deflection it has a converging effect and in the other direction of deflection a diverging effect. Therefore, an arrangement including a quadrupolar lens requires special steps to ensure that the electron beam is focused substantially on the target plate for both directions of deflection.

In a known arrangement including a magnetic quadrupolar lens for deflection amplification, this is achieved in that a rotation-symmetrical electron gun produces an electron beam which is focussed by a non-rotation-symmetrical focussing lens comprising at least one quadrupolar lens. This arrangement has various disadvantages. Due to the presence of one or more quadrupolar lenses of the focussing lens, the cathode-ray tube of the arrange ment must have a comparatively great length. The combination of the rotation-symmetrical electron gun with the non-rotation-symmetrical focussing lens yields a comparatively poor beam quality, that is to say that the dimensions of the beam in the plane of deflection and on the target cannot be equally small.

The invention has for an object to provide an arrangement in which these disadvantages are mitigated.

In an arrangement having a cathode-ray tube in which an electron beam produced by an electron gun comprising at least a cathode, a first grid and a last grid acting as an acceleration electrode is focussed substantially on a target by means of a focussing lens, which arrange ment includes means for deflecting the electron beam in a first direction, means for deflecting the electron beam in a second direction substantially at right angles to the first direction and a quadrupolar lens for deflection amplification, according to the invention, the first grid has an opening to compensate for the astigmatic effect at the quadrupolar lens which opening has a minor axis which is parallel to one of said directions of deflection and a major axis which is parallel to the other of said directions of deflection and said opening being symmetrical with respect to said axes.

Due to this opening in the first grid, longitudinal beam cross-overs are obtained in the electron gun. It may even be achieved that only one longitudinal beam cross-over is formed in the electron gun. Viewed from the focussing lens, the electrons apparently originate from two virtual objects, which will be referred to as virtual beam crossovers. The relative matching of the proportioning of the electron gun and of the quadrupolar lens is such that both virtual beam cross-overs are imaged on the target. In order to obtain a circular beam spot on the target, this matching must be such that the two virtual beam crossovers are imaged on the target in a magnification which is inversely proportional to the length of these virtual beam cross-overs. Due to the occurrence of longitudinal beam cross-overs, the increase in beam cross-section in the electron gun due to space charge is smaller than in a rotation-symmetrical electron gun in which one punctiform beam cross-over is obtained, and a beam of high quality can be obtained. Furthermore, the cathode-ray tube may be of small length because a short focussing lens is suflicient, which may also be rotation-symmetrical.

The focussing lens is preferably a rotation-symmetrical focussing lens. Such a lens can be manufactured in a simpler manner and be centered more readily than a nonrotation-symmetrical focussing lens.

The opening in the first grid may be of different geometric forms having two orthogonal axes of symmetry. This opening may be elliptical. An elliptical opening can be provided in the grid in a simple manner, for example, by punching. The provision of angular openings generally requires more complicated techniques, such as spark erosion.

In the simplest form, the electron gun is a triode gun comprising a cathode, a first grid and a last grid. The virtual beam cross-overs then correspond to two separate longitudinal beam cross-overs in the triode gun, i.e., one in the plane passing through the axis of the electron beam and through the minor axis of the opening in the first grid and the other in a plane passing through the axis of the electron beam and through the major axis of the opening in the first grid.

It should be noted that a triode gun comprising a cathode, a first grid having a non-rotation-symmetrical opening and a last grid acting as an acceleration electrode is known per se. This triode gun is used to produce an electron beam having a non-rotation-symmetrical cross-section.

Preferably, at least a second grid is interposed between the first grid and the last grid of the electron gun. Such an electron gun provides a larger freedom in the choice of the location of the virtual beam cross-overs than a triode gun. In the construction of an arrangement according to the invention, the order of succession of the virtual beam cross-overs may even be opposite to that in the triode gun. The opening in the second grid may have different geometric forms. This opening is preferably rotation-symmetrical so that the second grid can be readily manufactured and centered. The electron gun may be a tetrode gun comprising a cathode, a first grid, a second grid and a last grid. The spherical aberration in the tetrode Patented Feb. 24, 1970 axis of the electron beam and through the major axis of the opening in the first grid, which is of advantage with regard to the increase in beam cross-section in the electron gun due to space charge.

The invention further relates to a cathode-ray tube for use in an arrangement of the kind described above. In the arrangement, the focussing lens, the deflection means and the quadrupolar lens may be composed of electrostatic and/or magnetic means. In a favourable embodiment, these means all have an electrostatic effect and the invention more particularly relates to a cathode-ray tube for use in such an embodiment of the arrangement. This hausted envelope between the electron gun and the target rotation-symmetrical focussing electrodes, at least one electrode system, for example one pair of deflection plates for electrostatic deflection of the electron beam and electrodes for forming an electrostatic quadrupolar lens field for amplification of the said deflection. The assembly of the electron gun, the focussing lens, the deflection means and the quadrupolar lens can be readily mounted in such a cathode-ray tube.

The invention will be described with reference to the accompanying drawing, in which:

FIGURE 1 shows a cathode-ray tube according to the invention comprising a triode gun;

FIGURE 2 shows part of a cathode-ray tube according to the invention comprising a tetrode gun;

FIGURE 3 is a sectional View of the electron gun and the focussing lens of the tube shown in FIGURE 1;

FIGURE 4 is a sectional view of the electron gun and the focussing lens of the tube shown in FIGURE 2;

FIGURE 5 is a partial sectional view taken on the line VV of FIGURE 4;

FIGURE 6 illustrates diagrammatically the behaviour of the electron beam in a cathode-ray tube as shown in FIGURE 1, in which the path of a deflected electron beam is shown;

FIGURE 7 illustrates diagrammatically the behaviour of the electron beam in a cathode-ray tube as shown in FIGURE 1, in which electron paths in the horizontal plane are shown;

FIGURE 8 illustrates diagrammatically the behaviour of the electron beam in a cathode-ray tube as shown in FIGURE 1, in which electron paths in the vertical plane are shown;

FIGURE 9 illustrates diagrammatically the behaviour of the electrons in the electron gun of FIGURE 3, and

FIGURE 10 illustrates diagrammatically the behaviour of the electrons in the gun of FIGURE 4.

Referring now to FIGURE 1, reference numeral 1 denotes an envelope of a cathode-ray tube in vertical sectional view. The envelope 1 accommodates an electron gun 2, a focussing lens 3, means 4 for vertical deflection, a quadrupolar lens 5 for deflection amplification, means 6 for horizontal deflection and a target 7. The axis of the tube is denoted by 30. The electron gun 2 comprises a cathode 11, a first grid -12 and a last grid 13 constituted by the spout of an electrode 14. The focussing lens 3 comprises the rotation-symmetrical electrode 14 and the rotation-symmetrical electrodes 15 and 16. The means 4 for vertical deflection comprises horizontal deflection plates 17 and 18. The deflection amplifier 5 comprises four hyperbolic electrodes 19, 20, 21 and 22 for the formation of an electrostatic quadrupolar lens field. The means 6 for horizontal deflection comprises vertical deflection plates 23 and 24. The target 7 is constituted by a phosphor screen which is struck by the electron beam and on which the electric signals applied to the tube are repro- 4 duced. The electric supply leads and the fastening means of the various electrodes are not shown in the figure.

In FIGURE 2, reference numeral 101 denotes part of an envelope of a cathode-ray tube in vertical sectional view. This tube comprises an electron gun 102, a focussing lens 103 and means 104 for vertical deflection. The tube further successively comprises a quadrupolar lens, means for horizontal deflection and a target, which elements are similar to the elements 5, 6 and 7 of FIGURE 1 and are not shown in FIGURE 2. The axis of the tube is denoted by 130. The electron gun 102 is a tetrode gun and comprises a cathode 111, a first grid 112, a second grid 125 and a last grid 113 constituted by the spout of an electrode 114. The focussing lens comprises the rotati0n-symmetrical electrode 114 and rotation-symmetrical electrodes 115 and 116. The means 104 for vertical deflection comprises horizontal deflection plates 117 and 118.

FIGURE 3 is a vertical sectional view of the electron gun 2 and of the focussing lens 3 of FIGURE 1. The figure shows the opening 28 in the first grid 12 between the electron-emissive surface 26 of the cathode 11 and the spout 13. The opening 28 is elliptical. The major axis lies in the plane of the drawing.

FIGURE 4 is a vertical sectional view of the electron gun 102 and of the focussing lens 103 of FIGURE 2. The elliptical opening 128 in the first grid 112 is located opposite the electron-emissive surface 126 of the cathode. The second grid 125 has an opening 129. The shape of these openings is shown in the sectional view of FIGURE FIGURE 6 illustrates diagrammatically the deflectionamplifying effect of the quadrupolar lens. The focussing lens 3, the deflection amplifier 5 and the target 7 of the tube of FIGURE 1 are shown symbolically in FIGURE 6. An electron beam 32, which enters the focussing lens 3 along the tube axis 30, is deflected in vertical direction at the deflection point D. In the absence of the deflection amplifier 5, the beam would follow the path 31. Due to the presence of the deflection amplifier 5, the beam follows the path 33. For this purpose, a fixed positive voltage with respect to the beam potential is applied to the plates 20 and 22 (cf. FIGURE 1), while an equal negative voltage with respect to the beam potential is applied to the plates 19 and 21 (cf. FIGURE 1). The deflection amplifier has a diverging effect in the vertical direction and a converging effect in the horizontal direction. The astigmatic effect introduced by the quadrupole is compensated for in that an astigmatic beam enters the quadrupolar lens due to the presence of the nonrotation-symmetrical electron gun and of the rotation-symmetrical focussing lens. This is illustrated with reference to FIGURES 7, 8 and 9, which show the structure of the non-deflected electron beam.

FIGURE 7 shows symbolically the focussing lens 3, the deflection amplifier 5 and the target 7 and electron paths 34 and 35 in the horizontal plane. The focussing lens 3 and the deflection amplifier 5 are provided in the figure with the sign which symbolically indicates the converging eflect in the horizontal plane. The electrons apparently originate from the virtual beam crossover 36 and are focussed on the target 7.

FIGURE 8 shows symbolically the focussing lens 3, the deflection amplifier 5 and the target 7 and electron paths 37 and 38 in the vertical plane. The deflection amplifier 5 is provided in the figure with the sign which indicates the diverging eflect of the deflection amplifier in the vertical plane. The electrons apparently originate from the virtual beam cross-over 39 and are focussed on the target 7.

The course of the paths 34 and 35 of FIGURE 7 and of the paths 37 and 38 of FIGURE 8 in the electron gun is shown in one plane in FIGURE 9. The electron-emissive cathode surface 26 and the focussing lens 3 are shown symbolically in FIGURE 9. The electrons in the horizontal plane pass through a longitudinal beam cross-over 40 corresponding to the virtual beam cross-over 36. The

electrons in the vertical plane pass through a longitudinal beam cross-over 41 corresponding to the virtual beam cross-over 39.

FIGURE is analogous to FIGURE 9 and relates to an electron gun for a cathode-ray tube as shown in FIG- URES 2, 4 and 5. The electron-emissive cathode surface 126 and the focussing lens 103 are shown symbolically in FIGURE 10. The electron paths 134 and 135 in the horizontal plane and the electron paths 137 and 138 in the vertical plane are shown in one plane. The figure illustrates the specific beam properties which can be obtained in a tetrode gun. Viewed from the focussing lens 103, the electrons in the vertical plane apparently originate from the virtual beam cross-over 139. Viewed from the focussing lens 103, the electrons in the horizontal plane apparently originate from the virtual beam crossover 136. The order of succession of 136 and 139 is opposite to that of 36 and 39 in FIGURE 9. The paths 134 and 135 cross each other in the longitudinal beam cross-over 140, but the paths 137 and 138 in the vertical plane do not cross each other so that the increase in beam cross-section in the electron gun due to space charge is small.

What is claimed is:

1. In combination a cathode-ray tube in which an electron beam produced by an electron gun comprising at least a cathode, a first grid and a last grid acting as an acceleration electrode is focussed substantially on a target by means of a focussing lens, and means for deflecting the electron beam in a first direction, means for deflecting the electron beam in a second direction substantially at right angles to said first direction and a quadrupolar lens for deflection amplification, said first grid having an opening to compensate for the astigmatic effect of said quadrupolar lens, said opening having a minor axis which is parallel to one of said directions of deflection and a major axis which is parallel to the other of said directions of deflection and said opening being symmetrical with respect to said axes.

2. The combination as claimed in claim 1 in which said focussing lens is a rotation-symmetrical focussing lens.

3. The combination as claimed in claim 1 in which said opening in said first grid is elliptcal.

4. The combnation as claimed in claim 1 in which at least a second grid is interposed between said first grid and said last grid.

5. The combination as claimed in claim 4 in which said second grid has a rotation-symmetrical opening.

6. A cathode-ray tube comprising an elongated evacuated envelope, an electron gun at one of said envelope, a target at the other end of said envelope in the path of said electron beam, means between the electron gun and the target to focus the electron beam thereon, and means to deflect the electron beam in two directions perpendicular to one another, said electron gun comprising a cathode, a first grid, and a last grid for acwlerating electrons in said beam, said first grid having an opening therein to compensate for astigmatism in said electron beam, said opening having a minor axis which is parallel to one of said directions of deflection and a major axis which is parallel to the other of said directions of deflection, said opening being symmetrical with respect to said axes.

7. A cathode-ray tube as claimed in claim 6, characterized in that it successively comprises inside its exhausted envelope between the electron gun and the target rotation-symmetrical focussing electrodes, at least one electrode system for electrostatic deflection of the electron beam and electrodes for forming an electrostatic quadrupolar lens field for amplification of the deflection.

8. A cathode-ray tube as claimed in claim 7 in which the opening in said first grid is symmetrical.

9. A cathode-ray tube as claimed in claim 6 in which a second grid is interposed between said first grid and said last grid.

10. A cathode-ray tube as claimed in claim 9 in which the second grid has a rotation-symmetrical opening.

References Cited UNITED STATES PATENTS 3,376,449 4/1968 Harrison 313- 3,373,310 3/1968 Worcester 315-18 3,349,269 10/1967 Hamann 313-80 2,988,660 6/1961 Corpew 313-80 2,919,381 12/1959 Glaser 315-31 2,884,559 4/1959 Cooper 315-31 RODNEY D. BENNETT, JR., Primary Examiner JOSEPH G. BAXTER, Assistant Examiner US. Cl. X.R. 313-80; 315-18 3 33 UNIT ED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3.497.263 Dated Februarv 24. 1970 Inventor s) JAN HASKER It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 3, line 18 after "'Ihis insert --cathode-ray tube successively comprises inside an ex- Col. 5, line 42, "elliptcal" should read --elliptical--.

Signed and Sealed this 14th day of ly 1970.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents 

